This invention relates to a blanket shuffling method for a liquid metal fast breeder reactor (LMFBR).
LMFBR's, especially the heterogeneous core variety, have a design problem, explained below, which is termed "thermal striping". The basic source of this problem is the inherent difference between the power generation in fuel and blanket assemblies in a heterogeneous LMFBR core. A heterogeneous core is a core having a plurality of fuel and blanket zones interspersed throughout the core causing a multitude of blanket to fuel interfaces. The power generated in a fertile material fueled blanket assembly increases continuously with the breeding of fissile fuel, while the power generated in a fissile material containing fuel assembly decreases continuously during burnup. During its lifetime, a blanket assembly increases its power output by a factor of 2 to 5 before it reaches its design limits and must be removed from the reactor and replaced. The coolant flow rate through a blanket assembly is controlled by a fixed inlet orifice, the design of which is dictated by those limits which are approached at the end of life. Thus, a blanket assembly is overcooled over most of its lifetime, which for internal blanket assemblies in a heterogeneous core, is on the order of 2 to 3 years (same as fuel assembly lifetime). However, the overcooling in radial blanket assemblies is even more pronounced because of longer lifetimes (4 to 5 years) and higher power gradients across the assembly. That is, the coolant flow rate is set by the rod with the maximum power which may be as much as 5 times higher than that in the minimum power rod.
The effect of blanket overcooling is that at beginning of life, the coolant from a blanket assembly may be as much as 350.degree. F. cooler than that from an adjacent fuel assembly. If this "maximum potential" temperature difference were completely mitigated by coolant mixing, conduction and entrainment, there would be no thermal striping problem. However, flow testing of reactor models has demonstrated that large differences in assembly outlet temperatures result in hot and cold coolant streams impinging on surrounding structures. Temperature differences from 30 to 60 percent of the maximum potential were observed in flow patterns away from the outlet nozzles, in the Upper Internals Structure (UIS) and as much as 60 to 80 percent of the maximum potential was observed near assembly outlet nozzles. When the hot and cold flow streams impinge upon adjacent structures, thermal stresses, due to differential thermal expansion, are induced in these structures. If the stresses exceed the fatigue strength of the material, crack initiation and, if stresses are severe enough, crack propagation can occur. This is the problem called "thermal striping". For Type 316 stainless steel the limits on maximum fluid temperature difference are 80-120.degree. F. for permanent structures and 120-160.degree. F. for replaceable structures. As can be seen, large temperature differences on the order of 350.degree. F. violate these limits even with partial mitigation by mixing and conduction. Inconel 718 can be used to solve the problem because its design limits are approximately twice those for type 316 stainless steel, but its cost is higher. Thermal striping problems are especially severe in heterogeneous cores because of the high number of blanket fuel interfaces where the temperature differences occur. Consequently, it is desired to provide a method to mitigate thermal striping to such a degree that 316 stainless steel can be used for replaceable and permanent reactor structures, in an LMFBR having fuel and blanket regions comprising a heterogeneous core.